1. Field of the Invention
The present invention relates in general to electric vehicles, viz., the vehicles using an electric motor as a power plant, and more particularly the present invention relates to a device and a method for controlling the electric motor of the electric vehicles.
2. Description of the Prior Art
In order to clarify the task of the present invention, a conventional control device for the electric motor will be described with an aid of FIG. 1 which however shows an electric vehicle EV with a control device of the present invention.
The device has an inverter to convert DC power (viz., direct current power) from a battery "BAT" into AC power (viz., alternating current power) for controllably driving an AC electric motor used as a power plant of the electric vehicle.
In the control device, when, for starting the vehicle, a main key switch "MKS" is turned ON, relays "Ry1" and "Ry2" are turned ON and then an auxiliary relay "Ry4" is turned ON. Upon the ON-turning of the auxiliary relay "Ry4", a DC link capacitor "C1" starts charging of the electric power from the battery "BAT". Under this condition, due to presence of a resistor "R1", the charging current to the capacitor "C1" is controlled to a suitable lower level. When, with increase of the charging, a terminal voltage of the capacitor "C1" is raised up to a certain level, an inverter relay "Ry3" is turned ON. With this, the battery becomes directly connected to the capacitor "C1". Because, under this condition, the terminal voltage of the capacitor "C1" has been raised up to such a certain level, the direct connection of the battery "BAT" to the capacitor "C1" does not induce excessive current flow to the capacitor "C1".
In electric vehicles wherein a synchronous type is used as the motor, the motor serves as a generator when, under running of the vehicle, the main key switch "MKS" is kept OFF making the inverter inoperative. Thus, under this power off running, the capacitor "C1" can be charged by the motor (viz., generator) through diodes "D1", "D2", "D3", "D4", "D5" and "D6". The charging is so made that the terminal voltage of the capacitor "C1" depends on the running speed of the vehicle. Thus, it easily occurs that the terminal voltage of the capacitor "C1" is raised higher than that of the battery "BAT".
However, when, with the terminal voltage of the capacitor "C1" being higher than that of the battery "BAT", the main key switch "MKS" is returned ON (or reclosed), it tends to occur that excessive current is forced to flow from the capacitor "C1" to the battery "BAT" through the inverter relay "Ry3" and the main circuit relays "Ry1" and "Ry2", and/or excessive voltage is applied to an auxiliary equipment "AUX". The auxiliary equipment "AUX" shown in the drawing has one terminal connected to a line extending between the main circuit relay "Ry1" and the inverter relay "Ry3" and the other terminal connected to a line extending between the main circuit relay "Ry2" and one terminal of the capacitor "C1".
In order to avoid the above-mentioned drawbacks, various measures have been hitherto thought out, which are for example of a type to suppress the reclosing (viz., ON-returning) of the main key switch "MKS" unless the vehicle slows down to a certain lower level, a type to provide the control device with an electrically durable main circuit and a type to discharge the capacity "C1" through a resistor upon reclosing (viz., ON-returning) of the main key switch "MKS". However, even these measures have failed to exhibit satisfied results in obtaining easy operability, lower cost and downsizing of the control device.